Really. We've known for over 100 years that parts of the brain control their own blood supply depending on how much oxygen they need. We haven't really understood how different regions regulate the flow, but we're beginning to.

The large blood vessels supplying your brain branch to form the network of arteries covering its surface. Inside, the vessels branch into smaller arteries and continue branching until they reach your capillaries, which are only wide enough for one red blood cell to pass through at a time.

Throughout your body, smooth muscle that surrounds your arteries and arterioles regulate blood flow by decreasing or increasing the diameter of the vessel. But blood flow through capillaries is also regulated, even though there are no surrounding smooth muscle cells.

It turns out that an obscure cell type, called pericytes, that can squeeze are wrapped around the blood vessels at intervals do the job. Recent work shows they constrict and relax, changing the diameter of capillaries in response to changes in neuronal activity. This is handy arrangement, kinda like individual plants in your garden having a way of turning on their own irrigation.

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